Chicago mob boss Rocca manages to get Lester Gillis sprung from jail in Joliet. His motive is to have Gillis kill a labor organizer, but Gillis refuses, preferring to work with Rocca's gang on robberies instead. He meets mob moll Sue Nelson and they start a relationship. He is relaxing, alone in his hotel room, when cops burst in, finding a gun Rocca has planted to frame Gillis for the labor leader's murder. Gillis vows revenge, escapes from the cops with Sue's help, then guns down Rocca and two henchmen. He adopts Sue's surname as an alias.

In a holdup at a pharmacy, Gillis is winged by a gunshot. He goes to Doc Saunders (Cedric Hardwicke), whose patients include America's most wanted criminal, John Dillinger (portrayed by Leo Gordon). Acquiring a nickname, "Baby Face Nelson", a grateful Gillis joins up with Dillinger and quickly becomes the FBI's second most wanted man.

The ruthless Baby Face goes on a shooting spree, even killing innocent motorists just to steal a car. He doesn't like playing second fiddle to Dillinger, but after the arch-criminal is shot in Chicago, it becomes Baby Face's turn to be public enemy number one. He commits multiple murders, even killing Doc in a fit of anger, and frightens Sue by placing a rifle sight on children.

Trapped by a roadblock, Baby Face flees on foot and is shot several times. Stumbling to a graveyard, he pleads with Sue at first, then taunts her, to put him out of his misery, and she does.

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